1. Field of the Invention
The present invention relates to the field of permeable and semi-permeable woven and non-woven web drying, and in particular to reducing thermal stresses in a Honeycomb® shell for drying such webs.
2. Discussion of Background Information
Cylindrical Honeycomb® drying rolls, such as those described by U.S. Pat. Nos. 3,259,961, 3,590,453, and 4,050,131, are well known in the art and typically these rolls are large and expensive apparatuses. The air permeable shells of such rolls comprise alternating thin straight and thin undulated strips that extend between thick metal end members, thereby effectively forming a cylindrical shell comprised of corrugated layers. The undulated strips may form, for example, half-hexagonal or triangular-shaped openings between the straight strips.
During the drying process, heated air impinges on a wet web traveling on the rotating roll. The heated process air travels through the web and between the shell openings formed by the alternating layers of straight strips and undulated strips. Sometimes, deckle bands are applied to the outer surface of the shell. Deckle bands are thin, solid strips of material that align along one edge with an end member and wrap around the entire circumference of a roll, but extend across only a small portion of the roll width. The inboard edge of a deckle band therefore defines the sheet width to be processed by the roll.
Typically, moisture in the web cools the heated air so that the temperature inside the shell is cooler than that of the heated air applied to the web. In single tier shells, this cooled but still hot air travels radially through the shell and the undulated strips prevent any axial flow within the shell structure. In dual layered shells, this cooled, but still hot, process air travels through the shell, and because of the spacing between the tiers of corrugations, air can move axially along channels formed between the two layers of corrugations, i.e. the inner diameter corrugation and the outer diameter corrugation, which are spaced apart. If the width of the traveling web is less than the width of the roll, heated air passes through the exposed shell area without any cooling. In that case, the heated process air travels through the shell at maximum temperature and some of the heated process air flows axially along channels running along the length of the roll between the layers of undulated strips. Additionally, when the traveling web sheet width does not precisely match the width of exposed roll surface area as defined by a deckle band applied to the surface of a roll, maximally heated air flows into the axial channels formed between the deckle band and the outer diameter corrugation layer and between the inner and outer corrugation layers. Thus the portion of the shell under the deckle band and adjacent to the thick metal end member connected to the shell is heated by hotter air than the portion of the roll covered by the travelling web sheet.
During the drying process, the heated air typically ranges from 250 to 550 degrees Fahrenheit and cools to 180 to 320 degrees Fahrenheit after passing through the traveling web and picking up moisture. If the sheet width of the traveling web matches shell and/or deckle width, the thin strip divider and corrugation layers at the shell ends are exposed to the same cooled air that reaches the center portion of the shell. If the sheet width, however, fails to precisely match the width of the exposed shell, either taken alone or left uncovered by the deckle band, hot supply air leaks by the end of the sheet, entering the shell and running along the axial channel between at least the layers of bent strips (i.e. the spaced apart inner diameter and outer diameter corrugation layers).
This heated process air heats the thin strips and the end members. The thick end members, therefore, are exposed to process air on one side and much cooler ambient (e.g. 70-120 degrees Fahrenheit) air on the other side, and the thin shell structure therefore operates at a significantly higher temperature than the more massive supporting end members. This temperature difference causes the support structure to exert significant restraining forces on the shell, resulting in high stress conditions at and near the connection of the thin strips of the shell and the thick end members. Furthermore, when the end members are manufactured from materials having a smaller coefficient of thermal expansion than that of the thin strips, these stresses at the connection between the elements are exacerbated. The thin straight and undulated strips absorb heat and expand more rapidly than the thick metal end members. This causes a sharp thermal gradient at the intersection of the thin strips and thick end members and these steep thermal gradients cause high stresses. The magnitude of the temperature difference, the thermal gradient and the differences in coefficients of thermal expansion are all major factors the affect stress levels at and near the connection between the divider and end ring.
Similarly, thermal stresses occur during sudden warm up or cool down of a roll. To avoid such stresses, warm up and cool down durations are often extended so that the thin metal strips and end members absorb heat and cool more evenly and more gradually, thereby reducing the slope of the thermal gradient. This adds time and inefficiency to the overall manufacturing process for drying travelling webs.
A need therefore exists for a device that blocks axial airflow between layers of undulated strips forming at least a dual layered Honeycomb® shell and therefore reduces the thermal gradient between the shell and the end members, thereby reducing cycle fatigue and extending the useful life of the production roll.